Lithium-ion batteries have shown technical success and commercial growth since the initial work by Sony in the early 90's based on lithium insertion electrodes; essentially consisting of high voltage cobalt oxide cathode invented by J. B. Goodenough (U.S. Pat. No. 5,910,382 and U.S. Pat. No. 6,391,493) and carbon anode using coke or graphitized carbonaceous materials.
Since then, lithium-ion batteries have progressively replaced existing Ni—Cd and Ni-MH batteries, because of their superior performances in most portable electronic applications. However, because of their cost and intrinsic instability under abusive conditions, especially in their fully charged state, only small size and format cells have been commercialized with success.
Existing lithium-ion batteries rely on anodes made from graphite. However, the anode based on the carbonaceous material has a maximum theoretical capacity of only 372 mAh/g (844 mAh/cc), thus suffering from limited increase of capacity. Lithium metals, studied for use as the anode material, have a high energy density and thus may realize high capacity, but present problems associated with safety due to growth of dendrites and a shortened charge/discharge life cycle as the battery is repeatedly charged/discharged. Because of these disadvantages and problems, a number of studies have been conducted and suggestions have been made to utilize silicon, tin or their alloys as possible candidate materials exhibiting high capacity and capable of replacing lithium as metal. For example, silicon (Si) reversibly absorbs (intercalates) and desorbs (deintercalates) lithium ions through the reaction between silicon and lithium, and has a maximum theoretical capacity of about 4200 mAh/g (9366 mAh/cc, a specific gravity of 2.23) that is substantially higher than that of carbonaceous materials and thereby is promising as a high-capacity anode material.
Silicon-based anodes theoretically offer as much as a ten-fold capacity improvement over graphite. However, silicon-based anodes have not been stable enough to cycling for practical use. One way of improving the cycle performance of silicon-based anodes is to reduce the size of the particles in the material used in the fabrication of the anode. Coating of the particles in the material used with carbon has also been found beneficial. The smaller size helps to control the volume change and stresses in the Si particles. The carbon coating on the silicon surface acts like an electrical pathway so that even when there is a volume change, contact is not lost with the current collector.
Silicon is produced industrially by carbothermal reduction of silicon dioxide (quartzite) with carbon (coal, charcoal, petroleum coke, wood) in arc furnaces by a reaction that in an idealized form can be written as:SiO2+2C→Si+2CO
In industry, the available raw materials are not pure and the product generally contains other elements, such as Fe, Al, Ca and Ti. With pure operation and pure raw materials and electrodes, it is possible to obtain silicon with less than 1-2% percent of other elements. This product is traditionally called metallurgical grade silicon metal even though solid silicon is not a metal.
If higher purity is required, metallurgical treatments like gas blowing (dry air, O2, Cl2) may reduce alkaline species (K, Na, Mg, Ca, Al, Sr) at temperatures higher than 1410° C. Those species will either be volatized from the liquid metal surface or be physically separated in a slag phase. If transition elements such as Fe, Ti, Cu, Cr, Mn, V, Ni, Zn, Zr, etc. need to be reduced, directional solidification may be used. Another efficient method consists of finely grinding solid silicon and expose the intermetallic phases to acid (HF, HCl, H2SO4 or a mixture). With those metallurgical treatments, the silicon metal purity can reach 99.999% (5N purity level).
For higher purity, chemical vapour deposition of Si from precursor species like SiHCl3 or SiH4 is needed. The so-called Siemens process is a perfect example. This process can easily reach a 9N purity level.
Silicon-based anode materials can be prepared at low cost from solid crystalline ingots or micron size powders by conventional grinding process (jaw crusher, cone crusher, roll crusher, jet mill, etc.). Mechanical attrition process is one of the most used processes to produce fine particles. Industrial wet nano-grinding bead mill equipment is available commercially, which can be used to reduce particle size down to 10 to 20 nm; see for example WO 2007/100918 for lithium metal phosphate ultrafine grinding. These techniques are especially useful for high purity Si.
One significant improvement to the problem of low electronic conductivity of complex metal alloy anode powders, and more specifically of Si-based materials, was achieved with the use of an organic carbon precursor that is pyrolysed onto the anode material or its precursor to improve electrical conductivity at the level of the anode particles.
It is also known that the electrical conductivity of a silicon powder is improved by intimately mixing conductive carbon black or graphite powder with the Si powder or the Si-alloys before grinding. Such addition of carbon black or graphite powder involves usually relatively large quantities of C to achieve good connectivity and does not result in a good bonding of the C to the silicon-based material crystal structure. This intimate bonding is a characteristic that is judged to be essential to maintain contact despite volume variations during long term cycling.
The inventors are aware of the following documents that relate to the invention: WO 2012/000854 and WO 2012/000858 both of Scoyer et al., U.S. 2011/0244334 and 2011/0244333 both of Kawada, and WO 2008/067677 of Liang et al.
There is still a need for improved methods for the preparation of particulate silicon-based materials that allow for the fabrication of high electrochemical energy storage capacity anodes.